1. Field of the Invention
This invention relates to the field of digital signal transmission, and more particularly to oscillating different types of digital signals over a communications channel.
2. Background of the Related Art
Issues concerning digital signal transmission are prevalent in the field of video conferencing. An important issue is the allocation of limited transmission channel bandwidth for each of the video, data, audio and control signals which are transmitted over the communications channel during a video-data conference. Transmission channel bandwidth typically is expressed as a rate at which digital signals are carried over the channel, such as in bits per second (bps).
International Telecommunication Union (ITU) Recommendation H.320 (March 1993)--Narrow-band visual telephone systems and terminal equipment ("H.320") is a standard governing video conferencing over an Integrated Services Digital Network (ISDN). An ISDN typically includes two parallel communication channels, called "B" channels, each capable of carrying digital signals at a rate of, i.e. having a bandwidth of, 64 kilobits per second (kbps). The standard mandates that 16 kbps of the bandwidth of the first B channel be reserved for digital audio signals. Another 1.6 kbps on the first B channel is reserved for other signals, such as bitrate allocation signals (BAS), leaving a bandwidth of 46.4 kbps unassigned. The second B channel has 1.6 kbs of bandwidth allocated for signals such as BAS signals, leaving 62.4 kbps of bandwidth unassigned. Therefore, in conventional video conferencing configurations governed by the H.320 standard, a total of 108.8 kbps of channel bandwidth is unassigned and can transmit video.
Video conferencing under the H.320 standard requires transmission of digital signals at all times, i.e.., there can be no dead air. In typical video compression algorithms used in H.320 video conferencing, when there is little or no movement from frame-to-frame, relatively few digital video signals are generated. In order to maintain a constant flow of digital signals during such times, predefined "fill" digital signals are sent. Therefore, in typical H.320 video conferences, available bandwidth is wasted, i.e., it is occupied by more fill signals when there is little movement from frame-to-frame.
FIG. 1 shows a hypothetical timing diagram of conventional video transmission in a video conference under the H.320 standard over an arbitrarily chosen 140 millisecond (ms) time window. Compressed digital video signals typically are sent in bursts from a video encoder. As shown if FIG. 1, the video encoder has generated an initial 60 ms burst of video which typically is nearly simultaneously transmitted over the available video bandwidth across the ISDN. A period in which there is little or no movement follows as is apparent from the 10 ms idle period. Fill data is sent during the 10 ms idle period. Subsequently, there may be some movement causing generation of, for example, a 10 ms video period which is transmitted over the available video bandwidth. Thereafter, periods in which there is some movement, are followed by periods of little or no movement, for the remainder of the 140 ms time window. Although the time window chosen is strictly arbitrary, and the respective times of video generation and amount of video generated are hypothetical, they reflect possible video and fill data transmission behavior over an ISDN according to the H.320 standard. It should be understood that the respective times of video generation within a sequence of windows have an arbitrarily chosen duration and amounts of video generated, and hence the respective periods of video and fill data transmission, will vary on a window-by-window basis.
In an H.320 video conference with data (video-data conference), such as in application-sharing schemes, remote users share an application and enter information related to the sharing of the application, such as by keyboard and mouse input. Relatively large quantities of data, such as files, graphics and text in the format specified by ITU recommendation T.120, which falls under the umbrella of the H.320 standard, are transmitted across the communications medium along with the audio, video and control information transmitted during a conventional video conference. Typically, in such video-data conferencing configurations, a fixed amount of the total bandwidth available is reserved for T.120 data transmission. For example, bandwidth available for T.120 data transmissions in such video-data conferencing configurations might be one of 32 kbps or 48 kbps leaving, for example, 76.8 kbps or 60.8 kbps of bandwidth available to transmit video. Of course less or more bandwidth can be allocated for data transmissions in accordance with design criteria. Whatever the amount of bandwidth allocated for T.120 data, the remainder typically is used for video.